Mems device having a getter

ABSTRACT

A microelectromechanical system (MEMS) device includes a high density getter. The high density getter includes a silicon surface area formed by porosification or by the formation of trenches within a sealed cavity of the device. The silicon surface area includes a deposition of titanium or other gettering material to reduce the amount of gas present in the sealed chamber such that a low pressure chamber is formed. The high density getter is used in bolometers and gyroscopes but is not limited to those devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/779,042 entitled “MEMS DEVICE HAVING A GETTER” by Samarao et al.,filed Mar. 13, 2013, the disclosure of which is hereby incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to a microelectromechanical system (MEMS) deviceand more particularly to a getter for a MEMS device.

BACKGROUND

MEMS devices, including bolometers and gyroscopes, require that thedevices are sealed at very low pressures of from 1 to 100 μBar toachieve increased sensitivity and to thereby provide a good signal tonoise ratio. Typically these devices are micro fabricated on the MEMSwafer and are sealed inside a cavity that is formed as a recess in a capwafer. The MEMS and cap wafers are typically bonded together usingvarious wafer bonding schemes like eutectic bonding or thermocompression or glass frit bonding. In all of these bonding scenarios,the sealed cavity is approximately at the same pressure level as ispresent during the bonding of the wafer. The typical pressure levels toachieve bonding is carried out is approximately 1 mBar. As a result,there is a need to further reduce the pressure in the sealed cavity upto three orders of magnitude down to 1 μBar. Typically, “Getters” areused for this purpose. The inner walls of the sealed cavity are coatedat selective locations with a getter material. A getter material is onethat combines with the gas molecules in the cavity either chemically orvia adsorption and increases the level of vacuum (or decreases thepressure) in the cavity. At times these getter materials might requirean elevated temperature to “activate” gettering of the gas molecules.

The efficiency of a getter is strongly dependent on the surface areawhich is exposed to the gas molecules. The larger the surface area, themore gas molecules are adsorbed into the getter and thereby evacuatedfrom the chamber leading to lowered pressure levels. Since the availablesurface area on the inside of a sealed cavity is limited, the surfacearea for creating a getter having improved performance is limited. Deeptrenches on the interior surface of the sealed cavity using processessuch as Deep Reactive Ion Etching (DRIE) could be implemented toincrease the surface area to some extent. However, the need for moreefficient getters continues to exist as the MEMS packages continue toshrink in size (especially in Consumer Electronics). Of course, smallerMEMS packages have reduced surface areas and therefore there is lessarea in which the trench patterns can be formed by DRIE.

Consequently, there is a need for a getter which can reduce the pressurein the sealed cavity to desirable levels. By reducing the pressure inthe sealed cavity, better performance of the MEMS devices can beachieved.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-7 depict a procedure which can be used to form a getter in asilicon substrate used as an upper portion of a bolometer.

FIG. 8 bottom portion of the bolometer formed of a silicon substrate,ALD bolometers, and aluminum contacts.

FIG. 9 is a schematic cross-sectional view of an upper portion and alower portion bonded together to form a bolometer.

FIG. 10 is a schematic cross-sectional view of a bolometer having anupper portion without a getter.

FIG. 11 is a schematic cross-sectional view of a bolometer having anupper portion with a getter.

FIG. 12 is a schematic cross-sectional view of a bolometer illustratingthe adsorption of gas molecules by a getter.

FIG. 13 is a schematic top view of a bolometer showing potentiallocations for the getter or getters.

FIG. 14 is an image of depositing Alumina into porous silicon, e.g.using atomic layer deposition.

FIG. 15 is a schematic cross-sectional view of a top portion of thebolometer showing the porosification of a getter using the substrate asan anode and the platinum as a cathode.

FIG. 16 is a schematic cross-sectional view of a top portion of thebolometer showing the porosification of the getter using light.

FIG. 17 is an image of porous silicon having an increase amount ofsurface area for forming a getter.

FIG. 18 is an image of a plurality of trenches in silicon to increasethe amount of surface area for forming a getter.

DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the disclosure is therebyintended. It is further understood that the present disclosure includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the disclosure aswould normally occur to one of ordinary skill in the art to which thisdisclosure pertains.

The present disclosure relates to reducing the pressure within thesealed cavity of a MEMS device by providing selective porosification ofthe silicon surface within the sealed cavity. By porosifying the siliconsurface within the cavity, the amount of surface area is greatlyincreased. Once the increased surface area is formed, a coating, such asTitanium is applied to the porosified surface area where the Titanium isdeposited via Atomic Layer Deposition.

Titanium is known to be one of the better getter materials that isreadily available, efficient, and CMOS-compatible. The material is alsorelatively inexpensive and is readily deposited using Atomic LayerDeposition (ALD). By selecting the parameters of the porosificationprocess, the diameter of the pores achieved in the silicon substrate aremade as small as 2-5 nanometers (nm). In addition, the duration of theporosification process is selected to achieve a porous silicon layerhaving thicknesses of more than 100 micrometers (μm). Such high densitygetters provide benefits to a wide variety and types of devices thatdemand higher levels of vacuum for operation, even devices which are notconsidered to be a MEMS device.

Devices like MEMS bolometers rely on the absorption efficiency of theinfrared waves followed by a subsequent heating up of the thermistormaterial (in this case, Platinum) for enhanced responsivity andsensitivity. At relatively higher pressure levels, like 1 mBar, the heatloss through convection from the bolometer to the surrounding structureis very high. As a result, conversion of the absorbed infraredelectromagnetic energy into thermal energy is immediately lost to thesurroundings as heat, thereby reducing the amount of thermal energyneeded to make an accurate determination of the amount of absorbedinfrared electromagnetic energy. Since a bolometer uses the thermalenergy to provide a change in resistance (i.e., thermistor), any loss ofthermal energy reduces the accuracy of the reading. However, at very lowpressures like 1 μBar, the heat loss due to convection is significantlyminimized and the only possible mechanisms for heat conduction at suchlow pressure levels are conduction and radiation, with radiation beingsignificantly smaller than conduction. When the pressure within thecavity is significantly reduced, the thermistor behavior is easilysensed by the interfaced external circuitry and a higher responsivity isachieved from the bolometer.

According to one embodiment, a method of fabricating a semiconductordevice includes forming a cavity in a first substrate, and forming apassivation layer within the cavity. A getter location is defined withinthe cavity at which the substrate is exposed through the passivationlayer. The substrate is then porosified at the getter location to formpores. A getter is then deposited on the getter location such that thegetter enters the pores and coats surfaces within the pores. Thepassivation layer is then removed.

The first substrate may comprise a cap wafer that is subsequently bondedto a second substrate with the cavity facing the second substrate. Thegetter is activated to reduce pressure within the cavity. In oneembodiment, the getter comprises titanium. The titanium is depositedusing an atomic layer deposition process and any titanium deposited onthe passivation layer is removed along with the passivation layer. Inone embodiment, the getter location is porosified to form pores havingsizes within in a range from 2 nm up to 2 μm. The pores may be formedusing an electrochemical etching process or by exposing the getterlocation to light.

Bonding contacts may be formed on the first substrate around the cavityand on the second substrate for bonding the substrates together using abonding process which forms a seal around the cavity. In one embodiment,the bonding process is used to activate the getter. In one embodiment,the bonding contacts are formed of aluminum and germanium so that anAl-Ge eutectic bonding process may be used. The first and the secondbonding contacts are exposed to a bonding temperature during theeutectic bonding process, and the bonding temperature may be used toactivate the getter. The second substrate may include a MEMS device,such as bolometer sensor. The bolometer is positioned within the cavitywhen the substrates are bonded together. In this embodiment, the firstsubstrate comprises a cap wafer for the bolometer.

FIGS. 1-7 depict a process for forming a getter in a silicon substrate.The process begins with a substrate 10 as shown in FIG. 1. The substrate10 may comprise silicon substrate, or wafer. The silicon substrate 10 isetched by known techniques to form a cavity 12 in one side of thesubstrate 10 which will serve as a portion of a cavity, or gap, for thebolometer. Once the cavity 12 is formed, bonding contacts 14, 16 areformed on the surfaces of the substrate portions 18, 20 surrounding thecavity 12. In one embodiment, the contacts 14, 16 are formed ofgermanium which is deposited on the portions 18 and 20. As discussedbelow, the germanium may be used for Al-Ge eutectic bonding.

Once the etched silicon substrate 10 includes the contacts 14, 16, apassivation layer 22 is formed on the exposed portions of the etchedsilicon substrate 10 and over the contacts 14, 16 which will be used toshield these areas during subsequent porosification processing. Thepassivation layer 22 may be formed by depositing a material, such assilicon nitride, photo-resist or other passivation material, onto thesubstrate. The passivation layer 22 may also be formed by doping. Forexample, the substrate may comprise a lightly p-doped silicon wafer. Apassivation layer may be formed by lightly or heavily doping thesubstrate with a p-type dopant. This results in good selectivity so thatduring subsequent processing, e.g., by electrochemical etching, onlyp-doped areas are porosified.

As can be seen in FIG. 4, the passivation layer is formed on the bottomsurface of the substrate portions 18, 20, over the contacts, 14, 16, andon the side walls and base of the cavity 12. A portion 24 of the base ofthe cavity is intended for porosification and is exposed through thepassivation layer 22. The portion 24 may be exposed in any suitablemanner depending on the type of passivation used.

Referring to FIG. 5, once the passivation material has been deposited,the substrate 10 is processed in a manner that porosifies the portion 24of the substrate, e.g., by causing the formation of pores and/or byincreasing the size of pores in the portion 24. In one embodiment, toporosify a silicon substrate, an electrochemical technique is appliedusing a platinum electrode 30 as a cathode and the silicon substrate 10as an anode. Through application of the electrochemical technique, adefined portion of the silicon substrate 10 becomes porosified to form aporosified portion 32. The desired range of pore diameters is from 2 nmup to 2 um.

Once the porosified portion 32 is formed, titanium is deposited on thebottom surface of the substrate and in the cavity 12 which forms atitanium layer 36 over the passivation layer 22. The titanium depositedonto the porosified portion 32 enters the pores and coats the exposedsurfaces within the porosified portion 32 which is why the titaniumlayer 36 does not appear over the portion 32 in FIG. 6.

Referring to FIG. 7, the titanium layer 36 and passivation layer 22 arethen removed from the substrate to expose the contacts 14, 16 and thewalls of the cavity 12. As a result, a silicon cap wafer 40 is formed,having a getter 32, which can be bonded to a second substrate 50 (FIG.8) which forms the bottom portion of the bolometer. As depicted in FIG.8, the substrate 50, which may comprise a silicon substrate and/or aMEMS wafer, includes bonding contacts 52, 54, e.g., aluminum bondingcontacts, which are configured to bond with the contacts 14, 16 on thecap wafer 40. The contacts 52, 54 are formed on an oxidelayer/passivation layer on the substrate 40. The substrate 50 includesone or more thin, flexible membranes 60, e.g., formed using an ALDprocess, which are suspended a distance above the layer 56 of thesubstrate 50 and form one or more bolometer sensors.

The cap wafer 40 is bonded to the bottom portion 50 by positioning thecavity 12 over the bolometers 60 and bonding the bonding contacts 14, 16on the cap wafer to the bonding contacts 52, 54 at locations 61, 62 onthe substrate 50. In one embodiment, the bonding contacts 14, 16 areformed of germanium and the bonding contacts 52, 54 which allows Al-Geeutectic bonding to be used. As a result, the bolometers 60 are enclosedand sealed within a cavity formed between the cap wafer 40 and substrate50.

The performance capabilities and responsivity of a bolometer depends inpart on the pressure sealed within the cavity which affects heat lossvia convection. To minimize heat loss via convection, the pressuresealed within the cavity is made as low as possible. The sealed pressureis typically controlled by the bonding techniques used to bond thewafers together which results in the sealed pressure correspondingtypically to the pressure within the bonding chamber. For example, FIG.10 illustrates a capped cavity without the titanium getter 32 asindicated by the circled portion 64. Without the getter 32, the cavityis sealed at approximately the same pressure as the bonding chamberduring the Al-Ge eutectic bonding. Typically these values are around 1mBar. To increase bolometer responsivity, lower sealed pressures arerequired that are a few orders of magnitude lower than 1 mBar, e.g.,down to 1-100 μBar.

A cap wafer 40 including a getter 32 in accordance with the disclosureenables the sealed pressure within the cavity to be decreased during andafter bonding. Referring to FIG. 11, the getter 32 formed of titaniumand porous silicon acts as a very high density getter material. Thegetter 32 combines with the gas molecules in the cavity eitherchemically or via adsorption and increases the level of vacuum (ordecreases the pressure) in the cavity. Because the silicon has beenporosified, instead of a thin film of Titanium acting as a getter, theTitanium coated onto a very high surface area material, like poroussilicon, provides the Titanium covered high surface area. Because ofthis high surface area coated with titanium, a contacting surface isexposed to an increased number of gas molecules in a given time in thecavity and leads to a much lower pressure level thereby acting as anefficient getter.

FIG. 12 illustrates the mating of the upper portion 40 to the lowerportion 50 through applied pressure and heat. Once the internal cavityof the bolometer is sealed, the getter 32 adsorbs the gas molecules. Anygetter material, including titanium, typically needs a temperature of300 to 500 ° C. to activate “gettering”. Eutectic Al-Ge bondingtemperatures are 435° C. to 475° C. and are applied for a period of 1-2hours which allows the bonding process to serve as the getter activationstep as well.

FIG. 13 is a schematic top view of a bolometer showing potentiallocations for the getter or getters. The getter or getters arepositioned such that a getter does not block the exposure of the ALDbolometer pixels to the infrared rays. For instance, ALD bolometers 60are not blocked by a getter 70 and a getter 72. A dotted lineillustrates a cavity recess boundary 74 in the cap 40. An aluminiumgermanium bonding ring 76 is shown outside the cavity recess boundary74.

Deposition of titanium into the mesoporous or nanoporous siliconmaterial cannot be accomplished using conventional deposition techniqueslike chemical vapour deposition (CVD) or physical vapour deposition(PVD) techniques. However, ALD remains a viable technique for coatingporous silicon deep into its pores as small as 10 nm in diameter. As anexample and as illustrated in FIG. 14, ALD Alumina has been coatedsuccessfully for as deep as 30 μm into a porous silicon substrate usingthermal ALD as shown at location 80. In addition, partial diffusion hasbeen observed in a remaining 20 μm of porous silicon as shown atlocation 82. Bulk silicon is shown at location 84. In other embodiments,precursor species like Tetrakis-Dimethyl-Amido Titanium (or TDMAT) areused for ALD of Ti into Porous Silicon. Both Plasma-Enhanced and Thermalversions of ALD are also utilized.

FIG. 15 is a schematic cross-sectional view of a top portion of thebolometer showing the porosification of the getter using the substrateas an anode and the platinum as a cathode as previously described. Thismethod is also called electro-chemical etching/porosification. In FIG.16, the getter is shown being formed through the application of light.In this embodiment, “electroless” or photo-assisted porosification isemployed. Instead of using the substrate as an anode and a platinumelectrode as Cathode as shown in FIG. 15, light is directed onto thesubstrate to stimulate carrier generation thereby facilitatingsubsequent porosification of silicon. This method is also calledphoto-assisted etching/porosification. Using photo-assistedporosification, substrates having the lowest doping can be porosified.This provides an advantage, as the wafer does not hinder infra-redtransmission from the outside to the sensor element.

As an alternative embodiment to porous silicon as shown in FIG. 17, aplurality of sub-micron wide trenches 90 (see FIG. 18) are etched insilicon to mimic a relatively high surface area. The amount of surfacearea provided in the embodiment of FIG. 18, however, is not as high asporous silicon where pores are as small as 2 to 5 nm. Once the trenchesare formed, titanium deposited inside these trenches using ALD or othertechniques. In other embodiments, a combination of trenched structuresand the described porosification process is employed. (first trench,then porosify the surface of the trenched structure.

Those of skill in the art will recognize that the process described withreference to FIGS. 1-9 in other embodiments is modified to provide avariety of configurations designed for the particular embodiment.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe disclosure are desired to be protected.

1. A method of fabricating a microelectromechanical system (MEMS)device, the method comprising: forming a cavity in a first substrate,the first substrate comprising a silicon substrate; forming apassivation layer within the cavity; defining a getter location withinthe cavity at which the substrate is exposed through the passivationlayer; porosifying the substrate at the getter location to form pores;depositing a getter on the getter location such that the getter entersthe pores and coats surfaces within the pores; removing the passivationlayer; bonding the first substrate to a second substrate with the cavityfacing the second substrate, the second substrate including a MEMSdevice which is positioned in the cavity when the first substrate isbonded to the second substrate; and activating the getter.
 2. (canceled)3. The method of claim 1, wherein the getter comprises titanium.
 4. Themethod of claim 3, wherein the titanium is deposited using an atomiclayer deposition process.
 5. The method of claim 4, wherein any titaniumdeposited on the passivation layer is removed with the passivationlayer.
 6. The method of claim 4, wherein the pores are sized within in arange from 2 nm up to 2 μm.
 7. The method of claim 6, wherein the poresare formed using an electrochemical etching process.
 8. The method ofclaim 6, wherein the pores are formed by exposing the getter location tolight.
 9. The method of claim 3, further comprising: forming firstbonding contacts on the first substrate around the cavity; formingsecond bonding contacts on the second substrate at positionscomplementary to the first bonding contacts; and performing bondingprocess to bond the first bonding contacts to the second bondingcontacts such that a seal is formed around the cavity.
 10. The method ofclaim 9, wherein the bonding process activates the getter.
 11. Themethod of claim 10, wherein one of the first bonding contacts and thesecond bonding contacts comprises germanium, wherein the other of thefirst bonding contacts and the second bonding contacts comprisesaluminum, and wherein the bonding process comprises an Al-Ge eutecticbonding process.
 12. The method of claim 11, wherein the first and thesecond bonding contacts are exposed to a bonding temperature during theeutectic bonding process, and wherein the bonding temperature activatesthe getter.
 13. The method of claim 12, wherein the MEMS devicecomprises at least one bolometer which is positioned within the cavitywhen the first substrate is bonded to the second substrate. 14-19.(canceled)